Edit: M52 inline 6 Engine in E39

While taking a road trip this weekend I noticed that my engine coolant temp on my 1997 528i never got out of the blue on the highway. It stayed at the top end of the blue, or just out of it occasionally. At first I thought it was just because it is so cold out (It has been between -13 and +8 over the trip) however even after 2-3 hours on the road it never got warmer. I do not think this is normal behavior for my car, especially since we got in a 2002 530i later the same day and the coolant temp was smack in the middle after only a few mins. A 97 540 I drove (again the same day) also got warm just fine.

What do I need to check? I do not think it is normal for the car to be operating this low, even given the extreme cold temps. If the car is let to sit and idle for ~5mins it seems to get up to temp, but not on the road. I've also noticed a drop in fuel efficiency (above what is expected with the cold air temps). But I know my O2 sensors, MAF, and cats are all due to be replaced so I'm pretty sure the fuel consumption is related to that, rather than the temp.

Thanks!

The thermostat has failed and needs to be replaced. A cold running engine will rich and that will lower fuel mileage.

Thanks, thats kind of what I was thinking. Any suggestions for the best place to buy one (the dealer is a ways away, and way expensive)? I do most of my own maintance so I think I could handle the replacement myself provided it isn't terribly difficult.

About any autoparts should supply this, it pretty easy fix make shure you have the gasket or O ring depending on the sealing method.

Bought the part, will put it in tomorrow unless the temps stay below the single didgets outside again :-( Any special tricks or advice for installing a new thermostat? From RealOEM diagram it looks like it just screws off the side of the radiator, swap, and replace.

From RealOEM diagram it looks like it just screws off the side of the radiator, swap, and replace.
I think you are looking at the aux fan thermoswitch. The part that you need to change is #7 in:
http://www.realoem.com/bmw/showparts.do?model=DD53&mospid=47585&btnr=11_1546&hg=11&fg=35

no, I'm looking at the right part (that part). I'm assuming the big grey part that whole assembly bolts to is the radiator, Its been 10 below here so I havn't gone out to poke around my engine bay yet :-)

The thermostat housing bolts to the end of the engine block.

darn, guessed the wrong large grey block all the hoses run between :-) Thanks for the heads up though, probably saved me 5 mins of poking around the engine bay in the cold, and when it's subzero out 5 mins is a LONG time :-)